1. Field of the Invention
This invention relates to a process for fabricating semiconductor devices and, more particularly, to a process for depositing a metal film on a desired area of a semiconductor substrate.
2. Art Background
The fabrication of semiconductor devices often requires the deposition of a narrow, e.g., a few microns in dimension, metallic strip in a precisely defined area on a semiconductor substrate. Additionally, these narrow strips often must be quite thick relative to their width. For example, when high frequency field effect transistors (FETs) are manufactured the gate must be narrower than two microns and must be positioned between two ohmic contacts which form the source and drain. The use of a gate wider than a few microns necessitates an excessive distance between the drain and source and makes the device impractically large while degrading electrical characteristics. Additionally, the gate is usually at least one micron high. If a thinner gate is used, its resistance would be unacceptably high and degraded device performance would occur.
Typically in fabrication of devices such as FETs, the area of a substrate to be metallized is first delineated by covering all of the substrate except this area with a protective material, i.e., a delineating material. For example, a substrate is covered with a photoresist material which is exposed and developed to remove the resist from the selected areas of the substrate. The impracticality of aligning a resist or other delineating material relative to a previously deposited narrow metal layer such as is found in FETs requires that all metallization be done in a single processing step. That is, the entire metallization process must be completed before the delineating, e.g. photoresist, layer is removed.
Other problems arise from the impracticality of delineating material realignment. For example, as the delineated area of the substrate is metallized, metal is also deposited on the original delineating material. This layer of metal when thicker than one micron, as necessitated for certain devices, e.g. gates in FETs, substantially hinders the removal of this delineating material by typical solvent technique. Since, as discussed previously, metal layers and thus the deposits on the delineating material are often thicker than one micron, solvent techniques are usually unavailing for removal of the resist layer after metallization. Therefore, other removal techniques must be employed. However, other techniques, such as using a chemical reactant which removes the metal from the resist, also remove the metallized layer in the delineated area. To protect the desired metallization requires the positioning of a second resist or other delineating material onto the metal in the defined area. As discussed, however, the repositioning of a resist layer is not practical for metallizations having narrow widths.
A lift-off technique has been developed for use in fabricating narrow metallized areas. In this technique, a material which delineates the area to be metallized is deposited on an appropriate substrate. Aluminum is then deposited by conventional techniques such as evaporation onto the substrate. This deposited aluminum area is not controlled precisely. Not only the delineated area of the substrate, but also the entire surface of the delineating material is coated with a metallic layer. For substantially all common metals, this metallic layer covering the delineated material would prevent its removal where the metallization is greater than about one micron thick. (Thicknesses less than one micron as previously mentioned are unacceptable for many applications.) However, aluminum does not form a continuous film. Cracks in the aluminum surface allow introduction of a suitable liquid to remove the underlying delineated material.
In a typical process, a photolithography resist, such as AZ resist, is deposited on a semiconductor substrate to delineate a desired area for metallization. Aluminum is then evaporated onto the substrate surface. The AZ resist and its aluminum coating is then removed by using a solvent for the resist such as acetone.
Although aluminum metallization is essential for the lift-off process, there are many drawbacks associated with its use. For example, aluminum is very reactive and, therefore, acids cannot be used in any of the subsequent processing steps. Aluminum also reacts with gold to form an alloy composition which has unacceptable electrical properties for semiconductor devices. This reaction prevents the use of gold electrical contacts to the metallized aluminum layer. Since gold is a preferred contact in semiconductor technology, this is also a limitation. Additionally, most conventional techniques for depositing aluminum lead to a somewhat grainy morphology. This graining when severe leads to destruction of the device when a suitable electrical potential is applied.
Despite the problems associated with the use of aluminum, this metal continues to be used because of its unique adaptability to the lift-off process. Although other metals would be preferable in many semiconductor devices their impermeability for typical metallization thicknesses makes the use of the lift-off process unavailing. Thus, when the dimensions of the surface to be metallized require a single deposition step, without realignment, the inconvenience of aluminum metallized layers has been necessitated.